This invention relates to improvements in art of digestion of cellulosic material such as wood chips, and more particularly to a process and apparatus for removing the delignified chips from a digester at the completion of cooking.
In a conventional batch process for digesting wood chips, a digester is filled with chips and is charged with cooking chemicals. The digester is then sealed, and the temperature and pressure of the digester are elevated to desired cooking conditions. Elevated temperature and pressure are maintained for a cooking time period to achieve the desired delignification. During the cooking time, the cooking liquors may be circulated through the digester. At the conclusion of the cook, a blow valve at the bottom of the digester is opened, and the contents of the digester is discharged into a blow tank.
One common way of blowing the contents of a conventional batch digester is to open the blow valve leading from the bottom of the digester and leave it open, thereby permitting the liquor in the digester which is at an elevated temperature and pressure to flash into steam at the top of the digester and force the contents out of the digester.
In certain modifications to the conventional batch cooking process, the conventional blow technique utilizing liquor flashing to force the contents from the digester can not be used. For example, in one modification to the batch cooking process, displacement fluids, which may be fluids from subsequent washing or other process stages, are used to displace the cooking liquor from the digester before the digester is emptied. In this modification, the cooking liquors are displaced bottom to top substantially at cooking temperature and pressure, so that the heat energy contained in the cooking liquors can be utilized subsequently. The displacement liquors are at lower temperature than the cooking liquors so that, after displacement is complete, the contents of the digester, including delignified chips and the displacement liquors, are at temperatures substantially less than cooking temperature. In some such modifications, flashing may not occur or may be insufficient to empty the digester.
It is common in such modified batch cooking processes to utilize a fluid pumped into the top of the digester to force the digester contents through the blow valve at the bottom of the digester. The fluid pumped into the top may be a liquid, pressurized steam or air. In yet a further modification, pumps are used to remove the contents of the digester through the blow valve. In all of these modifications, the practice has been to open the blow valve and leave it open until the digester is emptied.
Certain results have been experienced in blowing digesters utilizing the established practices of leaving the blow valve open, which, although undesirable, were believed to be inherent and unavoidable from the blow techniques used. For example, when air is used as the medium for effecting blow, foaming tendencies increase and sulfur-containing gaseous emissions may be higher than allowable standards. Each of these "side effects" is now believed to be the result of air entrainment in the digester contents, inherently resulting from the air blow. Procedures and apparatus used to compensate for these undesirable results, such as the use of antifoaming chemicals, and emissions control systems are, in some instances, quite expensive. Often the compensating procedures or apparatuses also have undesirable side effects, which are believed to be unavoidably necessary.
Effects on the pulp have also been experienced when other fluids are used to blow digesters. For example, variations in the stock consistency have been experienced. After only several minutes of conventional blow procedures, the stock leaving the digester becomes slushy and foamy, due to fluid entrainment. The later volumes of stock are found to be much more dilute than earlier volumes of stock. Dilution of the digester contents by the blowing medium necessitates the use of large volumes of fluid medium for emptying the digester, which again, though undesirable, has been deemed inherent to and necessitated by the blow techniques utilized.
Yet another phenomena experienced in emptying digesters, particularly in modified batch processes, is that the digester empties inconsistently. During some blows, a substantial volume of the digester contents will remain in the digester. This has necessitated the incorporation of shower mechanisms in the bottom of the digester to wash the digester contents from the sides of the bottom of the digester, further diluting the stock. Again, the remedy to this undesirable result has been one of compensation, rather than correction of the result itself. The incomplete blows were viewed as an inherent, unavoidable problem resulting from the blow technique, and compensation was accomplished at not insignificant expense.